European Patent Disclosure EP 0 643 294 A1 describes the detection of foreign substances in a textile material by illuminating the test product and measuring the light reflected by the test product such that the presence of a foreign substance can be concluded from a change in the reflected light. For detecting foreign substances that are darker than the test product, the test product is projected on a light background on a sensor and, to detect lighter foreign substances, it is projected on a dark background. As lighting elements, light-emitting diodes (conventionally abbreviated "LED") of a certain color, such as green or red, are used. If the light emitted by the light-emitting diode does not suffice to illuminate the textile material adequately at the material speed required, lighting elements are used that emit a higher quantity of light, such as lasers, flash bulbs, or incandescent bulbs. As an alternative provision for amplifying the emitted light, it is disclosed that the number of LEDs can be increased and that a plurality of LEDs of a certain color can be combined into a so-called multichip LED array. The change in the reflected light, from which the presence of a foreign substance is concluded, comprises a change in the total brightness of the reflected light.
However, the apparatus described in European Patent Disclosure EP 0 643 294 A1 has disadvantages. Since LEDs of a certain color, such as green or red, employ a very narrow wavelength range, certain foreign substances may cause no change in brightness, or may not cause a sufficient change in brightness, thus impairing the reliability of foreign substance detection. Lighting elements with a broader light spectrum, such as lasers, flash bulbs or incandescent bulbs, or with a multichip LED array instead of a single LED, require more parts and considerably more structural space. Yet in certain sensors, such as yarn sensors, only a very limited amount of structural space is available. Also, the energy consumption of an incandescent bulb that is used for instance as an alternative to a single LED is markedly higher.
International Patent Disclosure WO 98/33061 describes the use of different colors or wavelengths. Different colors or wavelengths are intended to prevent the inability to detect contaminated foreign bodies in the test product, or the ability to detect them only poorly. From this Patent Disclosure WO 98/33061 A1, it can be learned that the light intensity of a single LED may be unsatisfactory, and thus to amplify the emitted light quantity and hence, to amplify the electro-optical signal, that a plurality of LEDs should be used instead of a single LED. Using many lighting elements, or using lighting elements that emit light at high intensity, such as arc lamps, however, as already noted above, requires additional components and takes up a considerable portion of the only limited available structural space in a yarn sensor. The apparatus described in Patent Disclosure WO 98/33061 A1 employs a measurement of the total brightness of the reflected light.